Colored metal and method of making it



United States Patent 3,173,785 COLORED METAL AND METHOD OF MAKlNG ITSamuel J. Manganello, Penn Hills Township, Allegheny County, Pa,assignor to United States Steel Corporation, a corporation of New JerseyNo Drawing. Filed July 1'7, 1963, Ser. No. 295,812

4 Claims. (Cl. 75--201) This invention relates to a through-coloredsintered metal product and a method of making it. More particularly, itrelates to a cohesive, sintered mass of intimately dispersed minuteparticles of a metal and a colored inorganic compound.

Colored metal products are widely used in industry. Usually, they arecolored with paints, enamels, chemicals, plastics and othersurface-treating agents. These products are susceptible to scratching,chipping, abrasion, heat and wear, resulting in the destruction ormarring of the colored surface or exposure of the uncolored metal.

An object of the invention is to provide a throughcolored sintered metalproduct that is not susceptible to the above-described defects and maybe used in the general applications for sintered metal products.

A further object of the invention is to provide a method of making sucha product comprising admixing a major portion of a metal powder and aminor portion of a colored inorganic compound powder that is chemicallyand physically stable under the sintering conditions, compacting theadmixture and sintering it in a substantially inert atmosphere.

in carrying out my method, 1 preferably admix, in percent by volume,between about 60 and 80% metal powder, between about 20 and 40% of acolored inorganic compound powder, and between about 0 and of a suitablebinding and lubricating agent. Preferably, the admixture particlesshould all pass through a standard 100 mesh screen. The metal powdersmay be pure or prealloyed, such as those used in conventionalpowder-metallurgy techniques. It is preferred to use metals that cantolerate large amounts of non-metallic inclusions, for example, suchface-centered-cubic pure metals as nickel, copper, aluminum and theprealloyed austenitic stainless steels and bronzes. However,body-centered-cubic metals such as iron and plain carbon steels may alsobe used for products that have static or low-stress applications, suchas decorative and ornamental sections.

The colored inorganic powders usable in my invention include oxides,nitrides, carbides, mixtures thereof and other refractory materialswhich retain color and are stable in admixture with the above-describedbase metals at sintering temperatures. These powders may be single ormixed compounds or mixtures such as the fri-t mixtures or othercommercial, proprietary mixtures used as coloring agents for vitreousenamels and ceramics.

I have found that an important guide to the chemical and physicalstability of a coloring agent with respect to a base metal, within thepreferred range of sintering temperatures, may be determined from tablesor curves plotting the standard free energies of formation of thecoloring materials against temperature. Thus, for example, where ametal-oxide coloring powder is used with an ironcontaining base metal,such as iron, one of the austenitic stainless steels, or a carbon steel,it is only necessary to find the iron plus oxygen line in a chartshowing the standard free energies of formation of the oxides per grammole of oxygen. These charts for carbides, oxides, phosphides,silicides, nitrides and sulphides are given in various references, suchas Thermochemistry for Steelmaking by J. F. Elliott and M. Gleiser,volume 1, published 1960, by Addison-Wesley Publishing Co., Inc,Reading, Mass.

Thus, within the preferred sintering temperature range between about1500 and 2200 F., a colored metal oxide in intimate contact with aniron-containing base metal will be reduced to its metallic state *as theiron in the base metal is oxidized, if the colored metal oxide has aless negative free energy of formation than iron oxide, i.e., afree-energy of formation curve above that for iron oxide in a chart ofoxides. Conversely, a colored metal oxide having a free energy offormation substantially more negative, i.e., a free energy of formationcurve below that for iron oxide, will probably be stable and retain itscolor during sintering. Thus, I have found that, when admixed with anaustenitic stainless steel and sintered at about 1850 F., red tungstenoxide and blue cobalt oxide having curves above that for iron oxide donot retain their color, rather becoming drab gray. However, greenchromium oxide and brown cerium oxide, having curves below iron oxide,do retain their colors and produce a through-colored product.

Accordingly, other stable oxides for use with an ironcontaining basemetal comprise the white aluminum and magnesium oxides, black titaniumoxide, green manganese oxide and yellow titanium and vanadium oxides. Aswith iron, some of the other metals occur in more than one oxide form,often exhibiting dilferent colors, as well as in mixtures of oxides.However, white powders, being so similar in color to silver-colored,basemetal powders, are not as effective as coloring agents foriron-containing base metals as in the darker-colored powders. Othercoloring materials, such as carbides or nitrides, can be similarlyinvestigated from other charts. Thus, golden tantalum carbide, blackboron carbide, yellow-brown titanium or zirconium nitrides, dark-bluetantalum nitride, and greenish-yellow magnesium nitride, would becoloring materials that may be successfully used with iron-containingbase metals. In like manner, other base metals and prospective coloringagents may be investigated in their ranges of sin-tering temperatures.Thus, coloring agents chosen from the hereinabove mentioned oxides,carbides, nitrides and mixtures thereof may be admixed vn'th other basemetals, compacted and sintered at the appropriate temperature, forexample, with nickel powder at about 1850 F., with Monel metal powder (anickel-copper alloy) at about 1750 F., with copper powder at about about1350 F. and with aluminum powder at about 950 F.

Proprietary coloring agents are often mixtures, for example, of oxides.Since their compositions are usually not disclosed, they would requiretesting to determine their chemical and physical stability in intimatecontact with base metals, under sintering conditions. The coloringagents should be stable to all constituents of a base metal, forexample, to chromium as well as iron, where a stainless steel is thebase metal.

The addition of nonmetallic binders and lubricants to the aboveadmixture often serves to improve the compressing characteristics of thecompacting step by reducing the interparticle friction and the frictionbetween particles and die Walls. Between about 0 and 10% of such anagent or agents may be added, for example, zinc stearate, polyethyleneglycol or talcum powder.

The admixture may be compacted at ambient or elevated temperatures andbefore or during the sintering step. After sintering, it may be coldrolled or otherwise worked. The compacting means may include pressing ina die or mold, roll forming, explosive or pneumaticmechanicalcompacting, or a slip-casting process, where a liquid dispersion of themetal powders is cast and subsequently sintered.

The compacted admixture may be sintered in a temperatureandatmosphere-controlled furnace. The sintering temperatures should not behigh enough to cause the coloring agent or base metal to change theirchemical or physical forms. The atmosphere should prevent oxidation ofthe base metal, reduction of the coloring agent and should not enhanceinteraction between the mixed powders. The temperature range may varybetween about 1500" and 2200 F. for iron-containing base metals, butwill vary according to the type of base metal and will usually be lowerthan when sintering only the base metal. For example, the range may bebetween about l500 and 2000 F. for iron or carbon-steel mixtures andbetween about 1800" and 2200 F. for the austenitic stainless steels. Forrelatively thin products, a sintering period of no more than about 20minutes is preferred.

The sintering atmosphere should be free of such components as oxygen,sulfur dioxide or hydrogen sulphide. Preferably the atmosphere should beinert. Such an atmosphere may comprise argon, nitrogen or helium. Thedew point of the atmosphere may range between about 60 and 60 F. Up toabout hydrogen may be tolerated in the atmosphere to prevent possibleoxidation of the base metal without reducing the coloring agent, if itis an oxide. Where hydrogen is present in the atmosphere, it ispreferred to operate on the high (wet) side of the dew-point range. Thesintered product is preferably cooled in the same inert atmosphere,usually for no more than about one hour.

A complete understanding of the invention may be obtained from thefollowing typical examples of process showing how the products are made.

EXAMPLE 1 Substantially equal spoonfuls of powders were measured out andadmixed in percent by volume to produce a mixture of about 70% of 310stainless steel, 25% green chromium oxide and 5% Zinc stearate.

ders all passed through a standard 100 mesh screen. A portion of themixture was poured into a 1% diameter by 3 high cylindrical mold to aheight of about 1". At room temperature, a pressure of tons per squareinch was applied to the mixture with a tight-fitting ram, the resultingdisc being about A thick. The disc was sintered in atemperature-gradient furnace for 16 minutes at about 1850 F. in an argonatmosphere having a dew point between about 5 and 5 F., and then cooledin another part of the furnace for 7 minutes in the same atmosphere. Thedisc exhibited a through-colored, cohesive mass of intimately dispersedminute particles of silvery stainless steel and green chromium oxide.The coloring appeared speckled; that is, particles of green oxide wereinterspersed between bonded, silvery, metal particles. Upon fracturing,cutting or grinding the disc, the color appeared uniform throughout thecross section.

EXAMPLES 27 Example 1 was repeated as Examples 2 to 7 inclusive,substantially similar conditions being maintained, except for theindicated changes. The conditions and the results obtained from theseven examples are summarized in Table I. In Example 3, the frit wasSolaramic frit, a product of Ferro Corp., Cleveland, Ohio. This is ahightemperature, vitreous-enamel frit believed to comprise a silicate orboro-silicate and is not in itself a coloring agent. The Example 3coloring material comprised green chromium oxide and about 10% of theSolaramic frit. In Examples 6 and 7, the P105 coloring agent was aproprietary product of Ferro Corporation sold as a coloring agent forceramics. The product is believed to comprise a mixture of calcinedoxides of iron, chromium and zinc and has a medium-brown color. Thiscoloring agent The powpowder was used as received from the manufacturer.

Table I Example 1 Example 2 70% 310 stainless steeL 25% chromium oxide.5% zinc stearate 40 70% 310 stainless steel. 25% cerium oxide. zincstearatc.

7. Silver and brown.

Example 3 Example 4 Volume Percent Base MetaL. Volume Percent ColoringAgent Volume Percent Binder Compacting Pressure, Tons/sq. in SintcringTemperature, F Sintering Time, minutes Sintcring Atmosphere AtmosphereDew Point, F Cooling Time, Minutes Final Color of Product, Particlesof..-

65% 310 stainless steel..- 30% chromium oxide and frit. 2% talcum powderNitrogen and 23% hydrogen. 0 to 20 15 Silver and green 70% electrolyticiron. 25% chromium oxide.

5% talcum powder.

5. Nitrogen.

Silver and green.

Example 5 Example 6 Example 7 Volume Percent Base Metal.... 316stainless .et e

Volume Percent Coloring 30% chromium 30% P105 Agent. ox e.

Volume Percent Binder 5% zinc stcaratc Compacting Pressure, Tons/sq. 30

lIl. Sintering Temperature, F Sintering Time, minutes G SinteringAtmosphere Atmosphere Dew Point, F. Cooling Time, Minutes Final Color ofProduct, Particles of.

Silver and green.

65% 316 stainless 310 stainless 5% zinc stearatm. 5% zinc steer-ate. 3O40.

60 Silver and gray Silver and brown.

The foregoing examples illustrate a preferred method and the productsobtained with a variety of base metals, coloring agents and conditions.In the Example 3 product, the Solaramic frit did not enhance the colorof the product, when compared with products made without its use. Incontrasting Examples 5 and 6, the mediumbrown proprietary coloring agentP105 turned gray, hence was not stable at a sintering temperature of2150 F., Whereas the green chromium oxide was stable. Example 7indicated the P105 material to be stable at an 1850 F. sinteringtemperature, since it retained its brown color.

The surface of the colored-metal product may be buffed or abraded tobring out more of the base-metal color and thus render the product morespeckled in appearance. Such treatment removes or fixes any excesscolored material on the surface that may tend to rub off duringsubsequent use. The strength and ductility of my products will be lessthan that exhibited by products of the sintered metal alone. For thisreason they are most useful for static or low-stress applications. Theirmechanical and physical properties may be varied by such changes as theproportions, ingredients and pauticle size in the mixture, the densityof the compact, sintering technique and subsequent work done thereon.

It will be evident from the foregoing that my invention provides novelthrough-colored sintered products and a method of making them.

Although I have disclosed herein the preferred practice of my invention,I intend to cover as Well any change or modification therein which maybe made without departing from the spirit and scope of the invention.

I claim:

1. A method of making a sintered ornamental product comprising mixing inpercent by volume between about 60% and 80% of a metal powder, betweenabout 20% and 40% of colored-compound powder, said compound being chosenfrom the group consisting of metal oxides, carbides, nitrides andmixtures thereof having a more negative free energy of formation at thesintering temperature than said metal powder and less than about 10% or"a binder, compacting said admixture, sintering said admixture at atemperature between about 1500" and 2200 F. in an atmosphere comprisedprincipally of inert gases chosen from the group consisting of argon,nitrogen, hciium and containing not more than about 5% hydrogen,maintaining the dew point of said atmosphere between about -20 F. and 20F., and then cooling the sintered product in said atmosphere to producea sintered product wherein colored specks of said compound are uniformlydispersed throughout the metallic, silver-colored matrix and contrast incolor therewith.

2. A method as defined in claim 1 characterized by maintaining the dewpoint of said atmosphere between about 0 F. and 20 F.

3. A method as defined in claim 1 characterized by said admixturecontaining about 70% metal powder, about 25% colored-compound powder andabout 5% of said binder.

4. A method as defined in claim 1 characterized by said admixturecontaining between about and stainless steel, between about 25% and 30%oxide powder chosen from the group consisting of cerium oxide andchromium oxide and about 5% of said binder.

References Cited in the file of this patent UNITED STATES PATENTS2,479,914 Drugmand et a1 Aug. 23, 1949 3,012,951 Storcheim Dec. 12, 19613,047,383 Slayter July 31, 1962 3,061,756 Henderson Oct. 30, 19623,087,234 Alexander et al. Apr. 30, 1963

1. A METHOD OF MAKING A SINTERED ORNAMENTAL PRODUCT COMPRISING MIXING INPERCENT BY VOLUME BETWEEN ABOUT 60% AND 80% OF A METAL POWDER, BETWEENABOUT 20% AND 40% OF COLORED-COMPOUND POWDER, SAID COMPOUND BEING CHOSENFROM THE GROUP CONSISTING OF METAL OXIDES, CARBIDES, NITRIDES ANDMIXTURES THEREOF HAVING A MORE NEGATIVE FREE ENERGY OF FORMATION AT THESINTERING TEMPERATURE THAN SAID METAL POWDER AND LESS THAN ABOUT 10% OFA BINDER, COMPACTING SAID ADMIXTURE, SINTERING SAID ADMIXTURE AT ATEMPERATURE BETWEEN ABOUT 1500* AND 2200*F. IN AN ATMOSPHERE COMPRISEDPRINCIPALLY OF INERT GASES CHOSEN FROM THE GROUP CONSISTING OF ARGON,NITROGEN, HELIUM CONTAINING NOT MORE THAN ABOUT 5% HYDROGEN MAINTAININGTHE DEW POINT OF SAID ATMOSPHERE BETWEEN ABOUT -20*F. AND 20*F., ANDTHEN COOLING THE SINTERED PRODUCT IN SAID ATMOSPHERE TO PRODUCE ASINTERED PRODUCT WHEREIN COLORED SPECKS OF SAID COMPOUND ARE UNIFORMLYDISPERSED THROUGHOUT THE METALLIC, SILVER-COLORED MATRIX AND CONTRAST INCOLOR THEREWITH.